In the maintenance and/or training of both domestic and wild animals, it is frequently desirable to confine the animals within certain areas and/or to keep the animals out of certain areas. Above ground fences have been commonly employed for this purpose. In certain locations and areas, however, the use of above ground fences is impractical, is prohibited, or is otherwise not possible. One known alternate to above ground fences, where for any reason it is decided that such fences are not to be used for animal confinement or exclusion, is to provide an audible warning signal and mild electric shock to the animal when the animal approaches a boundary line defined by a loop of electrical wire surrounding the area.
This known method is taught for example in U S. Pat. No. 4,766,847. In accordance with the teachings of this patent, an electric antenna in the form of a loop of electrical wire is layed out to define the boundary of the confining/exclusion (hereinafter in both the descriptions and claims, confining/exclusion will be collectively referred to as "confinement") area. AC current is passed through the wire, generating a surrounding magnetic field at a sub-broadcast band frequency.
A receiver, tuned to the frequency of this field, is attached to the collar of the animal. The receiver includes an electronic circuit which senses the intensity of the magnetic field and generates a selected voltage signal when the detected magnetic field signal strength exceeds a predetermined level, corresponding to a desired fixed distance from the wire. This voltage signal causes a mild electric shock to be applied to the animal. In addition, the electronic circuit preferably produces a warning signal, in the form of a high pitched noise, to alert the animal prior to shock generations.
A typical arrangement of such animal confinement apparatus can be seen, for example, in FIG. 1, which shows a dog 10 wearing a collar 14 having a receiver 12 attached thereto. An AC power transmitter (not shown) is, for example, located within garage 16 of house 18, but may be located in any convenient location. The transmitter is preferably powered from the household AC outlet of 115 volts AC. Current carrying electrical wire is connected at both ends to the transmitter output terminals and laid out around the house to form a closed loop 20 of any desired shape, within which the dog 10 is to be confined. The two ends of the wire are preferably interwound in a twisted arrangement 22 within the confinement area to cancel out the magnetic field generated around each conductor within this area. The AC transmitter generates an AC current which passes through the loop 20.
As a consequence of the AC current in the electrical wire loop 20, an alternating magnetic field is generated in the vicinity thereof. The intensity of the magnetic field is inversely proportional to the distance from the electrical wire 20 for any given value of AC current in the wire. In addition, the intensity of the magnetic field, at a given distance from the wire 20, is directly proportional to the AC current in the wire. The receiver 12 includes circuitry for sensing the strength of the magnetic field, for comparing this strength to a threshold level of magnetic field strength, and for generating a warning signal followed by a mild electric shock when the signal strength exceeds this threshold level. This threshold level of magnetic field strength corresponds to a predetermined distance from the electrical wire 20 for a given loop. The current in loop 20 can be calculated by Ohm's law, i.e. by dividing the transmitter voltage by the impedance of the loop.
The method described in the aforementioned patent suffers from two related deficiencies. These deficiencies are a relatively high impedance in the loop 20 which increases the power consumption to operate the apparatus and the fact that this impedance varies with the size and shape of the loop, creating a problem in assuring that the magnetic field of the loop will be sufficient to trigger the receiver at a desired distance in all applications. While resistive impedance can be reduced by increasing the diameter of the wire used for the loop, resistive impedance has been found, in accordance with the teachings of this invention, to normally account for less than ten percent of the loop impedance. Because of the nature of the apparatus and the low frequencies at which the apparatus is operated, capacitive impedance is normally negligible. The remaining over ninety percent of the loop impedance is therefore inductive impedance which is relatively unaffected by change in wire gauge and which varies with changes in the shape of the loop. These loop impedance variations are often difficult to compute. The inductive impedance also increases as a more or less linear function of loop length, requiring that more current be applied to the loop to maintain the field required to trigger the receiver at a desired distance as the loop size is increased. The magnetic field strength and the operation parameters of the apparatus in this prior art method are, thus, loop-size and loop-shape dependent. Further, the increased power required to overcome the inductive impedance particularly for large loops is not desirable for a number of reasons. First, increasing power output, and thus power consumption, makes the apparatus more expensive to use. Since the apparatus is typically operated continuously, this cost increase can be substantial. Second, increased power output from the transmitter requires a greater capacity transmitter and/or a power amplifier, resulting in higher hardware costs, and bulkier installations.